Ch 61b Patella Flashcards
Medial Patellar Luxation in Small-Breed Dogs
Etiology and Pathogenesis
- MPL not present at birth in most cases, the skeletal abnormalities that predispose to this condition are
- Therefore, it is a developmental disorder
- underlying cause not entirely understood
underlying skeletal abnormalities
1.coxa vara (a decreased angle of inclination of the femoral neck)
2.diminished anteversion angle (relative retroversion)
- lead to complex sequence of skeletal changes
- presence of the patella exerts pressure on the trochlear groove during growth > adequate depth and width.
- absence of the patella leads to trochlear hypoplasia.
- Intermittent luxation and reduction may cause progressive wearing of the medial trochlear ridge
- considered inherited, affected individuals should not be bred
what are typical deformities of MPL? (9)
- malalignment of the quadriceps mechanism (medial)
- coxa vara (decreased angle of inclination)
- femoral varus and genu varum
- shallow trochlear groove with poorly developed ridges
- hypoplasia of the medial femoral condyle
- medial displacement of the tibial tuberosity,
- internal rotation of the tibia relative to the femur,
- proximal tibial varus,
- internal rotation of the foot
theory has been questioned because coxa valga was identified as a significant risk factor
tibial valgus, though considered to be a compensatory change
version of the femoral neck, also known as femoral neck anteversion (FNA) or femoral torsion, is the angle between the femoral neck and the femoral condyles:
Epidemiology and Pathophysiology of MPL
medial %? bilateral %?
concurrent CCLR%?
- medial, lateral, or bidirectional
- some luxation is traumatic > tearing/stretching of the lateral parapatellar joint capsule and/or fascia can lead to instabiliy
- 82% developmental
- medial in 95-98%
- 2%- 5% lateral
- female-to-male ratio was 1.5 : 1
- Bilateral in 50% to 65%
- complication secondary to CCLR Sx
-> rate of 0.018%, mostly in larger (≥20 kg) dogs - thesis by Putnam: primary changes in the hip joint (coxa vara and diminished anteversion angle) are proposed to influence development and eventual malformation in the remainder of the pelvic limb.
- mild, moderate, or severe lameness, associated with grade
- Skeletal deformities more severe as grades increase
- Lameness likely related to the degree of cartilage erosion from patella and the medial trochlear ridge
- bilateral cases > crouched gait with stifle joint hyperflexion and internal tibial rotation
- Concurrent CCLR 15% to 20% (older + chronic MPL), unclear whether this truly occurs secondary or simply a manifestation of CCL disease
CCLR may occur secondary > cruciate ligament is placed under increased stress because the quadriceps mechanism is ineffective in stabilizing the joint
Q-angle
- Deviation of the direction of force of the quadriceps femoris muscle
- MRI: origin of the rectus femoris muscle, the deepest point of the trochlear groove, and the attachment of the patellar ligament on tibial tuberosity
- normal = 10.5 degrees
- grade 3 = 36.6 degrees
Grading System for Medial Patellar Luxation
- Grade 1 luxation is commonly an incidental finding
- Grade 2 luxation typically causes intermittent lameness, internal tibial rotation with flexion causes the patella to luxate, mild femoral varus, tibial valgus, and internal tibial rotation
- Grade 3 luxation may be mild, moderate, or severe lameness, patella is continuously luxated during ambulation
- grade 4 luxation, marked varus and internal tibial rotation, debilitating; crablike posture and must be carried
- acute worsening of chronic lameness may be assocaited with CCLR
MPL diagnosis
- grading of individual cases is based primarily on physical examination
- rule out concomitant cranial cruciate ligament disease (effusion, thrust, draw)
- walk and a trot is performed to evaluate overall conformation and to screen for overt skeletal deformity
- patellar ligament can be followed proximally from its attachment on the tibial tuberosity until the patella is located
- deterimine reducibility and direction
- patella alta or baja
- depth of the trochlear groove can be estimated by palpation
- alignment of the quadriceps mechanism assessed
RADS
- document luxation and assess the degree of degenerative changes
- identify and quantify skeletal abnormalities in severe cases
- skeletal deformity is present, orthogonal views of the femur and of the tibia, in addition to orthogonal views of the stifle
- Skyline views of the femur trochlear
- consider CT for derformities
decision making for MPL
- grade 1
- grade 2
- grade 3&4
- immature
- all the abnormalities are identified so that individualized treatment is provided for each patient
- medialization of the tibial tuberosity and/or a shallow trochlear groove should be corrected
- potential for OA to progress/develop
grade 1
- with no associated clinical signs, treatment is conservative.
- If lameness develops, it should be reevaluated
grade 2
- significant clinical signs, surgery is indicated, criteria includes:
- (1) significant episodes of lameness lasting 2 to 3 weeks or longer
- (2) three or more significant episodes of lameness that occur in a short time frame (1 month)
- occasional, mild lameness is not as straightforward (monitor for lameness, pain, OA progression)
grade 3 or grade 4
- surgical correction is warranted early
- to mitigate progressive skeletal deformity and osteoarthritis
very young patient with significant growth potential
- challenging
- Bony reconstructive techniques could result in damage to physes (avoid if significant growth potential)
- two-stage repair should be considered
- soft tissue reconstruction techniques +/- trochlear chondroplasty
- trochlear recession, TTT, and osteotomy of the femur once skeletal maturity
OA develops in MPL
MPL surgery (3)
- based on realignment of the quadriceps mechanism and stabilization of the patella within the trochlear groove
1. Extensor realignment by transposition of the tibial tuberosity, derotation of the tibia and/or correction of femoral varus.
2. patella stabilized in the trochlear groove by deepening and widening the groove
3. soft tissue balance by release of contracted tissues and by imbrication
Trochleoplasty
- goal: approximately 50% (or slightly less) of the patella protrudes above the trochlear ridges
- Histologic examination: cell loss and minor fibrillation confined to upper layers + no change in glycosaminoglycan content
- findings support the use of surgical techniques that preserve the hyaline cartilage
Trochlear Sulcoplasty
- abrasion trochleoplasty
- articular cartilage and several millimeters of subchondral bone are removed
- initially filled with well-vascularized, highly cellular, loose connective tissue > reorganized into fibrocartilage
- results in complete loss of the hyaline cartilage
- study: muscle atrophy, palpable crepitus, and severe erosion of the articular cartilage of the patella as early as 4 weeks following trochlear sulcoplasty
Trochlear Chondroplasty
dogs younger than 6 months
- articular cartilage can be gently separated from the underlying subchondral bone
- elevated with a periosteal elevator, and several millimeters of subchondral bone is removed with a curette, rongeur, or rasp
- can be difficult to achieve
Trochlear Wedge Recession
- geometric theory of similar triangles
- osteochondral autograft is developed from the trochlear sulcus
- cut is made slightly axial to the peak of each trochlear ridge to just proximal to the intercondylar notch.
- defect is deepened
- wedge fits exactly, achieving immediate stability, recession of articular surface, and a relative increase in height of ridges.
- Retropatellar pressure and congruence between the cut surfaces create suitable friction to stabilize the wedge without the need for internal fixation
- defect can be deepened in various ways: two additional osteotomies to remove a V-shaped piece of bone or only one on one side
- saw blade kerf may be sufficient to achieve adequate recession
fine-tooth, thin-kerf hobby saw blade, sagittal saw
principles of recession technique
- must be wide enough to accommodate the patella, or the patella will ride along the trochlear ridges
- wedge must have the same apex angle to achieve stability of the wedge via press-fit (subsequent osteotomies are performed parallel to the firs)
- depth of the trochlear groove should accommodate approximately 50% of the depth of the patella
- articulates with the patella throughout its normal range of motion
- minor discrepancies in angulation, the apex of the osteochondral wedge can be removed
- dislodging the wedge because it may fall from the surgical field
what to do with droppped osteochrondral graftd
- Five minutes of cleansing with a 10% povidone-iodine solution followed by a normal saline solution rinse appears to provide the optimal balance between effective decontamination and cellular toxicity for dropped autologous bone in the operative setting.
- provided decontamination in 10.4% of the incidents.
- up to 70% contamination rate in human surgery
Trochlear Block Recession
- to maximize preservation of hyaline articular cartilage
- abaxial margins of the rectangular osteochondral autograft are defined
- angled approximately 10 degrees axially, results in a press-fit
- from the proximal transtrochlear margin in the suprapatellar region to the distal transtrochlear margin near, but not entering, the intercondylar fossa
- osteotomy is initially directed perpendicular to the bone surface
- straight basilar osteotomy is performed to connect the proximal and distal transtrochlear margins
- alternately advancing from the proximal and distal margins > may help prevent fracture
- to deepen it: section from graft or from bed
- firmly pressed in place with a smooth-handled instrument to achieve a press fit
- proximal step defect is present and interferes with patellar tracking, it can be removed with a rongeur.
- The joint is copiously lavaged.
pro’s of block over wedge (4)
- increased proximal patellar depth,
- increased patellar articular contact with the recessed proximal trochlea,
- recession of a larger percentage of the trochlear surface area,
- greater resistance to patellar luxation in an extended position
experimental study
craniolateralization of TT (CrLT) may be advantageous in terms of the contact mechanics of the PFJ in canine MPL
Tibial Tuberosity Transposition
what k-wire mm?
- Medial displacement of the tibial tuberosity can be corrected
- assess patellar tracking: If the line of action of the patellar ligament is not centered on, and parallel to, the trochlear groove, then a tibial tuberosity transposition is warranted (typically obliquely directed)
- osteotomy should start at least 3 to 4 mm proximal to the attachment of the patellar ligament and should extend to the distal extent of the tibial crest
- approximately one-half of the depth of the tibial crest measured from the patellar ligament attachment to the cranial articular margin of the tibial plateau
- of sufficient size to be reattached with implants
- preferably leaving the distal periosteal attachment intact
- Flexion of the stifle joint will increase tension on the patellar ligament and will stabilize the tuberosity in the new position
- 2 x 0.035 (0.9 mm) to 0.062 (1.6 mm) Kirschner wire placed parallel in the widest portion of the tuberosity
- very small patients, one proximal to the other
- slightly distally and in a caudomedial direction (aiming distal to the fibular head)
- bone tunnel is drilled in the tibial crest 3 to 10 mm caudal and slightly distal to the distal extent of the osteotomy
- figure of eight pattern to create a tension-band wire; it is tightened with one or two twist knots
- alternative to FO8 wire: Kirschner wires within the tibial crest, or notch in the recipient bed of the tibia and fixation with a single Kirschner wire
pin and tension band
- FO8 effectively counteracts the distractive force of the quadriceps mechanism and is associated with a very low rate of implant loosening and/or Kirschner wire migration.
-
Zide 2020 – pin and tension band wire significantly stronger than pins alone for TTT
- pin orientation (level horizontal vs vertical) did not affect construct strength
- Hawbecker 2023 - 0° K-wire insertion of TTT → stronger than 30° when applied without TBW
- Cashmore 2014 - more caudodistal direction of K-wire → higher risk of TT avulsion
k-wires
soft tissue recon for MPL
- Retinacular release is the release of contracted retinacular tissues (fascia and other tissues) medially
- Capsulotomy is release of the contracted joint capsule
- release to proximal extent of the joint or beyond if necessary
- left open to prevent tension redeveloping; the synovial defect will quickly seal to prevent undue synovial fluid leakage
- For grade II/IV: the quadriceps femoris muscle can be released by making an incision: between the vastus medialis and the sartorius muscle, laterally between the vastus lateralis and the biceps femoris
- entire quadriceps mechanism can be freed of restrictive tissue connections
- ## Imbrication of soft tissues, joint capsue and or fascia, vest-over-pants (modified Mayo mattress) pattern; monofilament absorbable sutures
Antirotational Techniques
- mature animals, antirotational techniques are likely insufficient unless the underlying bony abnormalities are addressed concurrently
- consider doing for stage 1 immature repair: fabellotibial suture placed from the lateral fabella to the tibial crest results in external rotation of the tibia, thus moving the tibial tuberosity into a more lateral position
MPL in Large-Breed Dogs
- Etiology and Pathogenesis
- not entirely understood
- believed that coxa vara and a diminished anteversion angle are the underlying skeletal abnormalities
- coxa valga has been identified > bringing this theory into question
malalignment proposed to lead to:
- genu varum (bowlegged)
- distal femoral varus (bows toward midline)
- hypoplasia of the medial condyle
- external torsion of the distal femur
- a shallow trochlear sulcus with poorly developed ridges;
- proximal tibial varus or valgus
- internal tibial torsion
- medial displacement of the tibial tubercle
concurrent CCL in large-breed?
41%
The main difference between pathogenesis and pathophysiology is that pathogenesis is the process of how a disease develops, while pathophysiology is the study of the physiological changes that occur in the body as a result of a disease
Epidemiology and Pathophysiology
prevelence, incidence, L:P
- prevelence of medial vs lateral: small 98%, medium-size 81%, large breeds 83%, giant-breed 67%
- medial patellar luxation is still more common
- incidence in large-breed dogs appears to be increasing (40% in recent study)
- Evidence is insufficient to establish the exact sequence of events
- Abnormal medial tension of the extensor mechanism exerts uneven pressure on the distal femoral physis > femoral varus deformity
- **distal femoral varus **plays a significant role in medial patellar luxation in the large-breed dog
- compensatory proximal tibial valgus occurs > radiographic examination reveals a sigmoid skeletal structure
- patellar ligament length (L) to patellar length (P): significantly more proximal (patella alta) compared with normal dogs
- large-breed dogs with an L:P ratio greater than 1.97 were considered to have patella alta
- larger aLDFA was identified in the patellar luxation group
Rate of MPL complication secondary to the treatment of CCLD?
0.018%
diagnosis - common concurrent conditions to MPL (4)
- hip dysplasia
- CCLD
- angular or torsional malformation of the femur
- torsional or angular malformation of the tibia